linux-kernel - Re: [PATCH RFC tip/core/rcu 2/9] rcu: Provide cond_resched_rcu_qs() to force quiescent states in long loops

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Message-ID: <20140729172542.GK3935@laptop>
Date:	Tue, 29 Jul 2014 19:25:42 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
	dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com,
	fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com
Subject: Re: [PATCH RFC tip/core/rcu 2/9] rcu: Provide cond_resched_rcu_qs()
 to force quiescent states in long loops

On Tue, Jul 29, 2014 at 09:22:36AM -0700, Paul E. McKenney wrote:
> On Tue, Jul 29, 2014 at 09:55:36AM +0200, Peter Zijlstra wrote:
> > On Mon, Jul 28, 2014 at 03:56:13PM -0700, Paul E. McKenney wrote:
> > > From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> > > 
> > > RCU-tasks requires the occasional voluntary context switch
> > > from CPU-bound in-kernel tasks.  In some cases, this requires
> > > instrumenting cond_resched().  However, there is some reluctance
> > > to countenance unconditionally instrumenting cond_resched() (see
> > > http://lwn.net/Articles/603252/),
> > 
> > No, if its a good reason mention it, if not ignore it.
> 
> Fair enough.  ;-)
> 
> > > so this commit creates a separate
> > > cond_resched_rcu_qs() that may be used in place of cond_resched() in
> > > locations prone to long-duration in-kernel looping.
> > 
> > Sounds like a pain and a recipe for mistakes. How is joe kernel hacker
> > supposed to 1) know about this new api, and 2) decide which to use?
> > 
> > Heck, even I wouldn't know, and I just read the damn patch.
> 
> When Joe Hacker gets stall warning messages due to loops in the kernel
> that contain cond_resched(), that is a hint that cond_resched_rcu_qs()
> is required.  These stall warnings can occur when using RCU-tasks and when
> using normal RCU in NO_HZ_FULL kernels in cases where the scheduling-clock
> interrupt is left off while executing a long code path in the kernel.
> (Of course, in both cases, another eminently reasonable fix is to shorten
> the offending code path in the kernel.)
> 
> I should add words to that effect to Documentation/RCU/stallwarn.txt,
> shouldn't I?  Done.

No, but why can't we make the regular cond_resched() do this?
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